1. Field of the Invention
The subject invention relates to a process for the adsorptive separation of hydrocarbons. More specifically, the invention relates to a process for the continuous simulated countercurrent adsorptive separation of hydrocarbons.
The polyester fabrics and articles which are in wide use today are produced from a polymer of ethylene glycol and terephthalic acid. Terephthalic acid is produced by the oxidation of para-xylene. Para-xylene is typically recovered from a predominantly C8 aromatic hydrocarbon fraction derived from various sources such as catalytic reforming by liquid-liquid extraction and/or fractional distillation. The para-xylene is commercially separated from a para-xylene containing feed stream, usually containing all four C8 aromatic isomers, by either crystallization or adsorptive separation or a combination of these two techniques. Adsorptive separation is the newer technique and has captured the great majority of the market share of newly constructed plants for the production of para-xylene.
Essentially all of these adsorptive separation units use a simulated countercurrent movement of the adsorbent and the xylene containing feed stream. This simulation is performed using established commercial technology wherein the adsorbent is held in place in one or more cylindrical adsorbent chambers and the positions at which the streams involved in the process enter and leave the chambers are slowly shifted along the length of the beds. Normally there are at least four streams (feed, desorbent, extract, and raffinate) employed in this procedure. The location at which the feed and desorbent streams enter the chamber and the extract and raffinate streams leave the chamber are simultaneously shifted in the same direction at set intervals. Each shift in location of these transfer points delivers or removes liquid from a different bed within the chamber. This shifting could be performed using a dedicated transfer line for each stream at the entrance to each bed. However, this will greatly increase the cost of the process, and therefore the transfer lines are reused and each transfer line carries each one of the streams at some point in a cycle.
New and efficient chemical process technologies (e.g., XyMax®, Isomar®, PxMax®, and Tatoray® for the production of mixed aromatics, and Sarex XyMaxAE® for the production of mixed sugars) have presented the modern refiner with a dilemma of sorts, that is, how to accommodate the ever-changing availability of feedstocks of varying compositions in a process plant of relatively fixed architecture, without the need to perform major process piping and/or equipment revamp work.
2. Description of the Related Art
The general technique employed in the performance of a simulated moving bed adsorptive separation is well described in the open literature. For instance, a general description directed to the recovery of para-xylene was presented at page 70 of the September 1970 edition of Chemical Engineering Progress (Vol. 66, No. 9). A generalized description of the process with an emphasis on mathematical modeling was given at the International Conference on “Fundamentals Of Adsorption”, Schloss Elmau, Upper Bavaria, Germany, on May 6-11, 1983, by D. B. Broughton and S. A. Gembicki. U.S. Pat. No. 4,029,717 issued to F. J. Healy et al. describes a simulated moving bed adsorptive separation process for the recovery of para-xylene from a mixture of xylene isomers. Numerous other available references describe many of the mechanical parts of a simulated moving bed system, including rotary valves for distributing various liquid flows, the internals of the adsorbent chambers, and control systems.
U.S. Pat. No. 3,686,342 issued to R. W. Neuzil describes the separation of para-xylene from mixed xylenes using simulated countercurrent adsorption employing a zeolitic adsorbent and para diethylbenzene as the desorbent. This combination is a good representation of a commercial operation for this particular separation.
For purposes of explaining the transfer line apparatus employed by the present invention, reference is made to U.S. Pat. No. 3,201,491 issued to L. O. Stine and D. B. Broughton and International Application WO 95/07740. That art includes a recognition that the presence of residual compounds in the transfer lines can have some detrimental effects on a simulated moving bed process, and which art addresses the flushing of the line used to deliver the feed stream to the adsorbent chamber as a means to increase the purity of the recovered extract or sorbate component. The foregoing patents teach the use of only one feed stream and a line flush only through the one transfer line most immediately previously used to convey feed to the adsorbent chambers to avoid contamination of the extract stream with raffinate components of the feed remaining in this line when it is subsequently used to withdraw the extract stream from the chamber. The foregoing references employ a desorbent rich stream to flush the contents of this transfer line back into the adsorbent chamber.
U.S. Pat. No. 3,732,325 issued to Broughton is directed to an improvement to a simulated moving bed adsorptive separation process characterized as related to the recycle of extract from the extract product stream to the purification zone. This patent teaches the use of only one feed stream. Broughton further teaches the introduction of that single feed material stream to the bottom of the purification zone.
U.S. Pat. No. 4,031,156 issued to P. R. Geissler et al. is directed to an improvement to a simulated moving bed adsorptive separation process characterized as related to flush streams used in the process. This reference is directed to flushing the interstitial void spaces between adsorbent particles in the adsorbent chamber. This patent teaches the use of dual desorbent streams and also teaches the use of only a single feed stream.
U.S. Pat. No. 5,912,395 issued to Noe, directed to an improvement to a simulated moving bed adsorptive separation process, is characterized as related to flush streams used in the process. This reference is directed to the flushing of the transfer line most recently used to withdraw a raffinate material stream from the adsorbent chamber only with a single feed material stream. This reference does not teach means to accommodate more than one feed stream material.
Relative selectivity, (β), as used throughout this specification is defined as the ratio of the two components in the adsorbed phase divided by the ratio of the same two components in the unabsorbed phase at equilibrium conditions. The equilibrium conditions are determined when the feed passing over a bed of adsorbent does not change composition, in other words, when there is no net transfer of material occurring between the unabsorbed and adsorbed phases. Relative selectivity can be expressed not only for one feed compound as compared to another but can also be expressed between any feed mixture component and the desorbent material.
Where relative selectivity of two components approaches 1.0, there is no preferential adsorption of one component by the adsorbent with respect to the other; they are both adsorbed to about the same degree with respect to each other. As β becomes less than or greater than 1.0, there is a preferential adsorption by the adsorbent for one component with respect to the other. When comparing the selectivity of the adsorbent for component C over component D, a B larger than 1.0 indicates preferential adsorption of component C within the adsorbent. A β less than 1.0 indicates that component D is preferentially adsorbed leaving an unabsorbed phase richer in component C and an adsorbed phase richer in component D.
An important characteristic of an adsorbent is the rate of exchange of the desorbent for the extract component of the feed mixture materials or, in other words, the relative rate of desorption of the extract component. This characteristic relates directly to the amount of desorbent material that must be employed in the process to recover the extract component from the adsorbent. Faster rates of exchange reduce the amount of desorbent material needed to remove the extract component, and, therefore, permit a reduction in the operating cost of the process. With faster rates of exchange, less desorbent material has to be pumped through the process and separated from the extract stream for reuse in the process. Exchange rates are often temperature dependent. Ideally, desorbent materials should have a selectivity equal to about 1 or slightly less than 1 with respect to all extract components so that all of the extract components can be desorbed as a class with reasonable flow rates of desorbent material, and so that extract components can later displace desorbent material in a subsequent adsorption step.
In adsorptive separation processes, which are generally operated continuously at substantially constant pressures and temperatures to insure liquid phase, the desorbent material must be judiciously selected to satisfy many criteria. First, the desorbent material should displace an extract component from the adsorbent with reasonable mass flow rates without itself being so strongly adsorbed as to unduly prevent an extract component from displacing the desorbent material in a following adsorption cycle. Expressed in terms of the selectivity, it is preferred that the adsorbent be more selective for all of the extract components with respect to a raffinate component than it is for the desorbent material with respect to a raffinate component. Secondly, desorbent materials must be compatible with the particular adsorbent and the particular feed mixture. More specifically, they must not reduce or destroy the capacity of the adsorbent or selectivity of the adsorbent for an extract component with respect to a raffinate component. Additionally, desorbent materials should not chemically react with or cause a chemical reaction of either an extract component or a raffinate component. Both the extract stream and the raffinate stream are typically removed from the adsorbent void volume in admixture with desorbent material, and any chemical reaction involving a desorbent material and an extract component or a raffinate component or both would complicate or prevent product recovery. The desorbent should also be easily separated from the extract and raffinate components, as by fractionation. Finally, desorbent materials should be readily available and reasonable in cost.